1. Field of the Invention
The present invention relates to a vibration motor incorporated in various communication apparatuses (for example, mobile phones), game machines, or massaging equipment and to be used as a vibration generating source.
2. Related Background of the Invention
A vibration motor has, for example, an approximately cylindrical case arranged along a direction of an axis of rotation of a shaft and a weight is eccentrically fixed to a tip of the shaft protruding from a front end of the case. A rotor and stator are accommodated in the case. A coil of the rotor is connected to a commutator and a brush slidingly contacts with the commutator. A terminal, which is connected to the brush and protrudes out of the case, is in contact with a conductor on a printed wiring board to receive power. Then, a current flows from the terminal to the coil through the brush and commutator, torque of the rotor is generated to rotate the shaft, and the weight is swung around to cause vibration.
For a vibration motor surface-mounted on a printed wiring board, a metallic holder for holding a case may be provided to solder the vibration motor by reflow-soldering or the like. Vibration motors of this type include one (See Patent Document 1) that holds a cylindrical case with a pair of right and left bent springs rising up from a bottom which is in contact with a printed wiring board and provided in a holder, and one (See Patent Document 2) whose holder bottom is extended in the direction of the axis of rotation of a shaft to prevent detachment of the vibration motor from the printed wiring board.
[Patent Document 1] Japanese Patent Application Laid-Open No. 11-234943
[Patent Document 2] Japanese Patent Application Laid-Open No. 2005-198366
With increasing miniaturization of various kinds of precision apparatuses, further miniaturization of vibration motors incorporated in such precision apparatuses has been demanded as well. Since a vibration motor must be miniaturized while maintaining a gravity of the weight, if the size of the shaft in the direction of the axis of rotation becomes shorter, the position of a center of gravity of the vibration motor shifts, as a result, toward a front end side (weight side) of the case. Thus, for the vibration motor described in Patent Document 1 in which a terminal is provided on a rear end side of the case, the weight tilts forward if the vibration motor is simply put on a wiring board, increasing a risk of the terminal or a rear side of the holder being detached from the wiring board and making it difficult to provide suitable soldering. For the vibration motor described in Patent Document 2, on the other hand, the bottom of the case is extended to the weight side and thus the vibration motor is stable on the wiring board. However, since the extended bottom becomes a hindrance to the weight in the vibration motor described in Patent Document 2, the radius of rotation of the weight is limited, forcing downsizing of the weight.
An object of the present invention is to provide a vibration motor that enables upsizing of a weight in a centrifugal direction while stabilizing the vibration motor on a wiring board.